The present invention relates broadly to the field of optically activated atomic frequency standards and is more particularly directed to a cell construction therefor.
In optically activated atomic frequency standards of the prior art the atomic source composition, usually composed of a precise mixture of (i) resonance source atoms having a single electron in the outermost shell thereof, such as rubidium and cesium, and (ii) one or more inert buffer gases, is contained in a sealed glass cell. Often, the internal surfaces of the cell are coated with a material which is chemically inert with respect to the source atoms and which acts to mitigate against premature perturbation and/or relaxation of the excited resonance source atoms upon their impact with the walls of the cell. For instance, coating materials such as paraffins, polytetrafluoroethylene (PTFE) and dimethyldichlorosilane have been found to be helpful in this regard. The resonance source atoms are prepared by means of optical activation wherein light radiation of the appropriate wavelength and intensity is introduced into the cell. This can be achieved, for instance, by shining an appropriately filtered spectral light or a laser light beam of appropriate wavelength into the cell. The signal output of the cell can be represented by the light transmitted therethrough and/or by fluorescence emitted by the resonant source atoms contained therein.
Several problems attend the use of glass cells for the aforementioned purpose. For instance, the achievement of qualitatively and quantitatively precise and chemically uncontaminated mixtures of the atomic resonance source atoms and buffer gases within the cell is an essential to the achievement of a long lived properly operating optically activated atomic frequency standard. Such qualitative and quantitative precision is difficult to achieve utilizing a glass cell wherein the ultimate sealing of the cell after filling with the atomic source material is conventionally achieved by a glass melt or fusion tip-off wherein an integral glass inlet tubulation to the cell is sealed at essentially molten glass temperature. Too, even though the heating required to produce the glass tip-off may be relatively localized relative to the overall mass and/or size of the cell, the opportunity exists for sublimation or out-gassing of tramp or contaminant materials from the heated glass and into the resonant source material charge of the cell. In addition, glass cells of the prior art have been found to exhibit permeability to helium; thus, the atomic source material contained in such cells can be ultimately adulterated over time when the atmosphere in which they are operated contains helium. In addition, the internal environment of glass cells in optically activated atomic frequency standards of the prior art, and thus their accuracy and stability of operation, can be adversely affected by barometric pressure change. It is also known in the atomic frequency standard art that precise temperature control of an atomic resonance cell is often required to achieve acceptable results. Thus, in optically activated atomic frequency standards of the prior art it is conventional to provide the cell with various external insulation blankets and temperature controlled heaters in order to maintain constancy of internal cell temperature. Glass cells, due to the relatively low thermal conductivity of glass materials, in general, are somewhat slow to either exhibit an internal temperature change or to conduct externally applied heat therethrough at a sufficient rate as to allow quick adjustment and attainment of constancy of their internal temperatures.
In light of the present invention, the foregoing problems have been, severally or in combination, either fully resolved or at least substantially ameliorated. In addition, the present invention provides a novel cell construction in which additional benefits, some of which will be discussed hereinafter and some of which will be obvious, can be achieved.